The invention relates to diaphragm valves and more particularly to valves for fluid control of corrosive liquids.
In a diaphragm valve for fluid control of corrosive liquids such as acids or alkalis, the diaphragm is of an anticorrosive material, preferably of Teflon.RTM. polytetrafluoro ethylene [PTFE] manufactured by E. I. du Pont de Nemours and Co., Wilmington DL) which is, in thin sheet form, sufficiently flexible to be pressed against a seat in the valve body by a closure member known as "compressor", the latter being attached to the spindle end. Although the strength of the material is high, the teflon diaphragm requires a backing diaphragm of a polymer, such as natural or synthetic rubber, Viton or the like, which serves as a cushion pressing the PTFE-diaphragm evenly onto the valve seat.
The two diaphragms are in close contact and are secured by clamping both of them between the valve body and the bonnet, their position being fixed by holes in their corners coinciding with the holes in the valve body which serve to attach the bonnet to the body by through-going bolts and nuts. Connection between the teflon diaphragm and the compressor is conventionally made by a stud bolt having its head rigidly embedded in the center of the teflon diaphragm. Its shaft is provided, at its upper end, with a transverse hole in which a cross pin is rigidly fastened, so as to protrude out of both sides of the shaft by the same length. This pin engages with the bottom end of the compressor, in a known manner, by means of a bayonet closure in the form of a central hole and a slot crossing this hole positioned in the center of the compressor, the hole being widened inside the compressor body to permit an angular displacement of the pin in order to secure it in both upward and downward direction.
The backing diaphragm of rubber--which is generally much thicker than the teflon diaphragm--is positioned above the teflon diaphragm in close contact therewith, whereby the shaft of the stud penetrates through a cylindrical hole in its center. For closing the valve by turning the spindle, the compressor is pressed downwards onto the backing diaphragm which urges, in its turn, the teflon diaphragm onto the valve seat, the pin in the shaft end having sufficient clearance so as not being required to transmit the force from the spindle onto the diaphragm.